Genetic control of bacteriophage M13 DNA synthesis

Abstract

In the filamentous single-stranded DNA phage M13, the DNA replicates via the three steps shown in Figure 1: first, conversion of the infecting single-stranded molecule to a double-stranded replicative form; then, production of a pool of RF † † Abbreviations used: RF, replicative form; SS, single strand; MC, mitomycin C; CM, chloramphenicol; p.f.u., plaque-forming unit. molecules; finally, synthesis of progeny single strands. The various viral DNA molecules can be distinguished from each other when appropriately labeled nucleic acid extracts from infected cells are sedimented in neutral sucrose gradients. Phage genetic control over the three replicative steps was studied by infecting non-permissive host cells with amber mutants for each of the six known M13 genes and determining which mutations blocked steps in DNA synthesis. In addition, infections with the wild type phage were carried out in the presence of high levels of chloramphenicol to determine which steps could proceed without viral protein synthesis. The experiments show that no viral protein synthesis is needed to permit conversion of the infecting DNA strand to an RF molecule, but that the product of M13 gene 2 is required for the build-up of the RF pool and the product of gene 5 for progeny single-strand production. The scheme for M13 DNA synthesis differs in several respects from that for the icosahedral single-stranded DNA phages φX174 and S13.